Technical Field
The present invention relates to sintering of a glass preform for an optical fiber. The present invention relates to a sintering apparatus and a sintering method for a porous glass preform for an optical fiber which prevent leakage of gas included in a furnace core tube into a chamber, and intrusion of an atmosphere into the furnace core tube.
Related Art
Methods referred to as a VAD method and an OVD method exist as a method for manufacturing a glass preform for an optical fiber. In these methods, first, glass raw materials are burned in a flame to produce glass fine particles. The glass fine particles are made to adhere to a target rod to be rotated, in an axis direction or a diameter direction, to manufacture a porous glass preform. The porous glass preform thus manufactured is heated to 1400 to 1600° C. in an apparatus referred to as a sintering furnace, to produce a transparent glass preform. In advance of vitrifying the porous glass preform into transparent glass, a dehydrating treatment may be performed at 1000 to 1250° C.
The porous glass preform is heated in a container referred to as a furnace core tube and made of quartz glass. A heating furnace is disposed around a central part of the furnace core tube to form a heating zone. An opening part for inserting the porous glass preform into the furnace core tube and an upper lid for closing the opening part are placed on an upper end part of the furnace core tube. A shaft suspending the porous glass preform penetrates the upper lid. In order to seal a space between the furnace core tube and the upper lid, a sealing member is sandwiched between a flange of the opening part of the furnace core tube and the upper lid.
In a sintering process of heating the porous glass preform to vitrify the porous glass preform into transparent glass, the process is performed in an atmosphere mainly containing helium. To the atmosphere, dehydrating gas such as chlorine reducing an OH group in glass, or oxygen for reducing binding defect in the glass is added if needed. The atmosphere is heated to 1400 to 1600° C. during the sintering process. The reason why helium is used is that it has high heat conductivity and high solubility in the glass, and air bubbles are less likely remain in the glass.
The porous glass preform passes through the heating zone while being rotated and elevated/lowered, and thereby the porous glass preform is sequentially heat-treated from the end part to produce a preform for an optical fiber.
The heat-treated preform for an optical fiber is removed from the upper opening part of the furnace core tube by a glass preform elevating/lowering mechanism. However, when the preform for an optical fiber passes through the vicinity of the flange of the opening part, the furnace core tube and the flange of the furnace core tube are heated to 300° C. or more by the heat radiation of the preform for an optical fiber. At this time, since a sealing member such as an O ring inserted between the upper lid and the furnace core tube has a ring shape, the sealing member cannot be retracted from the opening part of the furnace core tube, and is in a state where the sealing member is still placed on the flange. Therefore, the sealing member is also exposed to a high temperature when the preform is removed.
A metal gasket is generally used for the sealing member used for a high temperature place in many cases. However, when the dehydrating gas such as chlorine is made to flow in the furnace core tube, the metal gasket is possibly corroded, and the flange of the furnace core tube made of quartz glass is possibly damaged, which makes it difficult to repeatedly use the metal gasket. Then, a heat resistant resin such as Viton or PTFE is used for the sealing member. However, the heat resistant temperature is at most about 300° C. The sealing member is cured and shrink-deformed by repetition use involving high temperature exposure when the preform is removed, which causes a decrease in sealability.
On the other hand, the temperature of the heating furnace while the preform is heat-treated is increased to 1600° C. as described above. The furnace core tube made of quartz has a refractive index of about 1.5, and serves as a light waveguide for a heat ray (infrared ray or the like) generated by the heat radiation near the heating furnace. For this reason, the radiation heat of the heating furnace propagates toward the flange of the upper opening part during the heat treatment of the preform. With the enlargement of the preform, the required heating value of a heater is increased, and a heat treatment time by zone heating is also increased. Thereby, the vicinity of the flange of the upper opening part is always exposed to a high temperature during the heat treatment of the preform, and the resin sealing member is heat-deformed by repeating the prolonged heat treatment, which causes a decrease in airtightness.
It is an object of the present invention to provide a sintering apparatus and a sintering method for a glass preform for an optical fiber which can prevent deterioration of a sealing member sealing a flange part of a furnace core tube of the sintering apparatus, suction of external air, and leakage of sintering atmosphere gas to the exterior.